A particular application for a data processing system is found in a flight data recorder. Flight data recorders are monitoring and recording instruments, carried aboard an aircraft, which systematically monitor and store the instantaneous values of various aircraft parameters. Early recorders were analog electromechanical devices which periodically marked, in analog form, the value of a given airplane parameter on a moving wire or other permanent storage medium. The time of occurrence of the parameter was also suitably scribed into the medium opposite the sensed parameter mark.
Subsequently, digital flight data recorders have been developed which operate by converting each analog aircraft parameter into a corresponding digital signal, and storing the digital signals on a permanent storage medium such as magnetic tape.
The numerous mechanical parts employed in the analog and digital type electromechanical flight data recorders have rendered such units expensive to construct and bulky in design, requiring periodic maintenance of the mechanical parts.
The development of solid state memory devices, such as electrically erasable programmable read-only memory (E.sup.2 PROM) or electrically alterable read-only memory (EAROM), has led to the design of all solid state flight data recorders. Solid state flight data recorders commonly employ a data acquisition system (DAS) which receives and processes the various aircraft input signals to be monitored and stored under the control of a central processing unit (CPU). Programming within the CPU controls the processing of input parametric signals to corresponding digital signals through the DAS and the subsequent transference of these ditital signals to controlled locations in the solid state memory.
A feature of the aforedescribed electromechanical flight data recorders is that additional data may be stored by these units simply by increasing the amount of storage medium as, for example, increasing the length of magnetic tape used to store signals. With solid state flight data recorders, however, the memory size is typically fixed by the number of solid state memory devices employed. In order to maximize the use of available solid state memory, therefore, it is desirable to provide data processing techniques which employ data compression to thereby reduce the amount of digital data required to be stored. Inasmuch as the storage and retrieval of data from a flight data recorder must be accomplished reliably, the data compression processing employed must be capable of reliable decompression to assure data integrity.